Soil Chemistry and Fertility
14 | Unit 2.2
i. alkalinity, alkali, salinity, and sodic soils
- Definitions
Alkalinity, soil: the degree or intensity of alkalinity in a soil, expressed by a value >7.0 for the soil ph
Alkali soil: (i) A soil with a ph of 8.5 or higher or with an exchangeable sodium percentage
greater than 0.15 (eSp>15). (ii) A soil that contains sufficient sodium to interfere with the
growth of most crop plants. See also saline-sodic soil and sodic soil
Saline soil: A nonsodic soil containing sufficient soluble salt to adversely affect the growth
of most crop plants. the lower limit of saturation extract electrical conductivity of such soils
is conventionally set at 4 dS m-1(at 25 ̊C). Actually, sensitive plants are affected at half this
salinity and highly tolerant ones at about twice this salinity.
Sodic soil: A nonsaline soil containing sufficient exchangeable sodium to adversely affect
crop production and soil structure under most conditions of soil and plant type. the
sodium adsorption ratio of the saturation extract is at least 13.
Saline-sodic soil: A soil containing sufficient exchangeable sodium to interfere with
the growth of most crop plants and containing appreciable quantities of soluble salts.
the exchangeable sodium ratio is greater than 0.15, conductivity of the soil solution,
at saturated water content, of >4dS m-1(at 25 ̊C), and the ph is usually 8.5 or less in the
saturated soil.
- Some elements contributing to alkalinity
Calcium: Calcium is common in many soils in arid areas. It helps the soil to form aggregates.
(Because it has a +2 charge (divalent), it can bind to two clay or organic particles). (Acid
demonstration in Demonstrations)
magnesium: Behaves similarly to calcium in helping to form aggregates.
potassium: An important plant nutrient
Sodium: toxic to plants at high levels
figUre 1. nUtRIent AVAILABILIty At DIFFeRent ph VALUeS. mAxImUm AVAILABILIty IS InDICAteD By WIDeSt pARt OF BAR.
Students’ Lecture Outline